buildkitd: configure telemetry providers with the global handlers

[jsternberg]

The global handlers are used as defaults for a bunch of OTEL functions and we'll likely use it that way in the future too. While it's always helpful to have a way to manually set the providers (and not rely on the global providers), setting the global providers from the main function is generally the correct thing to do.

This also removes the need for us to use the WithTracerProvider and WithMeterProvider option to the grpc stats handler as it will use the global configuration if this option isn't set. Since this code is in the main function too, it's safe to rely on that behavior.